In the past, one of the problems with sensing systems for tissue stimulators was the inability of the system to attenuate far field voltage artifacts. These artifacts were created by depolarization of body tissue in areas remote from the tissue the sensing system was adjacent. These artifacts were manifested as voltage potentials carried into a particular area of tissue to which the sensing system was adjacent. The sensing system would detect (sense) these voltages and interpreted them as depolarization events taking place in the local tissue when such depolarizations were above the threshold sensing voltage of the system. These far field signal voltages which surpassed the threshold voltage were input into the sensing section of the tissue stimulator. Such input signals could cause activation of certain pacing schemes for the stimulator.
The tissue stimulator which is of primary concern is a cardiac pacemaker. The lead systems associated with a cardiac pacemaker are either unipolar or bipolar. These lead systems fulfill two functions. The first function is to provide an electrical conduit by which an output pulse is delivered to stimulate the tissue to which the tip of the lead is adjacent. The second function is that it provides the sensing system to sense electrical activity as it takes place near the distal end of the lead system. The lead systems can be disposed in the atrium alone, ventricle alone, or both the atrium and ventricle. In present day lead systems, the atrial lead system usually comprises an atrial J-lead, while the ventricle lead system is a traditional straight lead.
In the past, whether there was a unipolar or bipolar lead system, there was a need for a sensing amplifier to amplify the sensed signal voltages before they were input into the sensing section of the tissue stimulator. In most cases the signal voltages would have voltage potentials in the millivolt range and amplification was mandatory. The signals which were amplified and input into the sensing circuit of the tissue stimulator indicated that there was a sensed signal voltage received by the unipolar or bipolar lead system. In most cases, only signal voltages sensed due to the depolarization of a tissue in close proximity of the sensing lead would produce a voltage great enough to overcome the threshold limitation of the amplifier circuit. These events would be either natural depolarizations of the tissue, depolarizations caused by an output pulse generated from the tissue stimulator, or the signal voltages produced by far field artifacts which were transmitted through tissue or body fluid to the tissue near the distal end of the lead system.
However, it was not desirable for the tissue stimulator to acknowledge and be responsive to such far field artifacts. The most desirous situation was to have a stimulator that would be responsive only to sensed depolarizations caused by natural P-waves or R-waves; or atrial or ventricular pulses output by the cardiac pacemaker in the tissue near the distal end of the lead system. If this was not the case and the tissue stimulator was responsive to far field artifacts viewing them as local depolarization of tissue, it could effect the pacing scheme of the tissue stimulator.
In order to alleviate the problem of sensing far field artifacts, present day tissue stimulators have blocking or blanking periods instituted in their programming to block atrial sensing after there is an atrial pulse or a sensed P-wave. This blanking period is generally long enough so that, if there is caused a retrograde signal transmitted back to the tissue near the distal end of the lead system it will not be sensed by the sensing system. Additionally, in situations where the tissue is that of the ventricle, and it is not desirable to sense the atrial depolarizations in the ventricle, there are blanking periods in ventricle sensing systems to prevent the sensing of such activity.
When the tissue stimulator is a cardiac pacemaker, there are situations in which there is a desire (or need) for it to remain uncommitted during the atrial/ventricle transmission time (AV delay). This AV delay is the normal time in which the atrial sensing system is blanked or blocked. In order to successfully carry this out, there must be a method by which to reject the retrograde transmission of the ventricular response to the atrial depolarization. The type of pacers in which this problem can arise are generally pacers that are triggered by an atrial depolarization. Therefore, pacers such as those pacing in modes AAI, AAT, VAT, and DDT will be activated by sensed far field signal voltages caused by the ventricular response to an atrial depolarization.
There have been studies of sensing systems comprising unipolar or bipolar lead systems. In an article by Parsonnet, Myers and Kresh, Characteristic of Intercardiac Electrograms II: Atrial Endocardio Electrograms, Vol. b3, July-Aug. 1980, pp. 406-417, the authors addressed investigations made relating to sensing systems which were unipolar and bipolar. In the article, the results of the testing revealed that problems were found regarding sensing of far field artifacts. In the article the authors produced a number of tables and graphs showing the representations of their findings. They did find that there was some attenuation of far field effects when a bipolar sensing system was used. However, there was no indication that they could control the amount of far field effects which were sensed by the sensing system. It was basically hit or miss whether there was sensing of the artifacts.
In their investigations, the authors used a standard sensing system, (with either bipolar or unipolar sensing systems) with standard sensing amplifiers for giving a certain amount of voltage gain to the sense signals. Their system was result oriented, not circuit oriented, and did not particularly take into account the characteristics of local depolarization wave fronts and far field artifact wave fronts. No apparatus at present has endeavored to use these characteristics to configure a circuit to attentuate far field artifacts. The present invention overcomes these problems noted in the foregoing.